Process for producing premium fischer-tropsch diesel and lube base oils

ABSTRACT

A process for producing a premium Fischer-Tropsch diesel fuel which comprises (a) hydroprocessing a waxy Fischer-Tropsch feed to remove the oxygenates that are present in the feed, whereby a first Fischer-Tropsch intermediate product is produced with reduced olefins and oxygenates relative to the Fischer-Tropsch feed; (b) separating the first Fischer-Tropsch intermediate product in a separation zone into a heavy Fischer-Tropsch fraction and a light Fischer-Tropsch fraction under controlled separation conditions; (c) hydroisomerizing the heavy Fischer-Tropsch fraction to improve the cold flow properties of the heavy Fischer-Tropsch fraction and recovering an isomerized heavy Fischer-Tropsch fraction; (d) mixing the isomerized heavy Fischer-Tropsch fraction with at least a portion of the light Fischer-Tropsch fraction of (b); and (e) recovering from the blend a Fischer-Tropsch derived diesel product meeting a target value for at least one pre-selected specification for diesel fuel.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of co-pending U.S.patent application Ser. No. 10/369,083 filed Feb. 18, 2003.

FIELD OF THE INVENTION

[0002] The present invention relates to the production of a premiumFischer-Tropsch derived diesel product produced by the blending of aFischer-Tropsch derived diesel fraction and a heavier isomerizedFischer-Tropsch derived base oil fraction to meet at least onepre-selected target property for the diesel product.

BACKGROUND OF THE INVENTION

[0003] Transportation fuels intended for use in diesel engines mustconform to the current version of at least one of the followingspecifications:

[0004] ASTM D 975—“Standard Specification for Diesel Fuel Oils”

[0005] European Grade CEN 90

[0006] Japanese Fuel Standards JIS K 2204

[0007] The United States National Conference on Weights and Measures(NCWM) 1997 guidelines for premium diesel fuel

[0008] The United States Engine Manufacturers Association recommendedguideline for premium diesel fuel (FQP-1A)

[0009] These specifications set a number of minimum technicalrequirements for diesel, so establishing a minimum quality level belowwhich the diesel fuel is not considered technically fit for the purpose.

[0010] Fischer-Tropsch derived transportation fuels meeting thespecifications for diesel fuels have certain advantageous propertieswhich make it possible to prepare a premium diesel fuel having very lowsulfur content and an excellent cetane number. However, due to theunique characteristics of Fischer-Tropsch derived syncrude additionalprocessing operations must be carried out to produce a suitable dieselfuel. Since Fischer-Tropsch derived products generally contain asignificant proportion of olefins, in order to improve the oxidationstability a hydroprocessing operation, such as mild hydrotreating, isusually necessary to saturate the double bonds. In addition, in order toimprove the cold flow properties of the fuel, the isoparaffin contentusually must be increased by a dewaxing step. Unfortunately, in thelarge volumes characteristic of transportation fuels, the cost of thedewaxing step may make the Fischer-Tropsch derived diesel fueluncompetitive with conventional petroleum derived diesel fuels.

[0011] Premium lubricating base oils may also be prepared fromFischer-Tropsch derived hydrocarbons, but due to the high proportion oflinear-paraffins in the product a dewaxing step also is required toimprove the cold flow properties prior to sale. However, lubricatingbase oils generally are produced in smaller quantities thantransportation fuels and have a higher commercial value, so the dewaxingoperation is not commercially impractical.

[0012] The present invention is directed to an integrated process whichis able to produce a premium Fischer-Tropsch derived diesel fuel incombination with a premium Fischer-Tropsch derived lubricating base oil.In the process of the invention, the properties of the base oil fractionrecovered from the syncrude are carefully controlled to produce aproduct which after further processing may be blended back into thediesel fraction to produce a diesel fuel having the desired properties.The process of the invention is advantageous because it is possible toproduce a premium diesel fuel without hydroisomerizing the entire dieselproduct. This decrease in feed results in significant savings in capitalcosts due to the smaller vessel size required for the isomerizationreactor. By significantly lowering the cost of processing theFischer-Tropsch derived diesel fuel, it is possible to produce a premiumproduct which is competitive in cost with conventional petroleum deriveddiesel fuel.

[0013] The Fischer-Tropsch syncrude fraction which is processed intodiesel fuels usually will have a boiling range between about 150 degreesF. (about 65 degrees C.) and about 750 degrees F. (about 400 degreesC.), typically between about 400 degrees F. (about 205 degrees C.) andabout 600 degrees F. (about 315 degrees C.). The majority of thehydrocarbons boiling in the range of diesel will contain between about 9and about 19 carbon atoms in the molecule. Lubricating base oils aregenerally prepared from that portion of the Fischer-Tropsch syncrudeboiling above about 600 degrees F. (about 315 degrees C.) and containingat least 20 carbon atoms in the molecule. However, the initial boilingpoint of the base oil fraction may be higher, for example about 750degrees F. (about 400 degrees C.). One skilled in the art will recognizethat there is considerable overlap between the upper boiling point ofdiesel and the initial boiling point of the base oil fractions. Theprecise cut point selected will depend upon the properties desired inthe final products. By carefully controlling the separation pointbetween diesel and base oil, it is possible to tailor the properties ofthe two products, so that when a portion of the hydroisomerized base oilis blended back into the diesel, the diesel product will meet thecriteria of a premium diesel fuel without the necessity of isomerizingthe entire diesel stream.

[0014] Naphtha which is also produced by the process of presentinvention has a boiling range below that of diesel but above that of thenormally gaseous hydrocarbons, such as butane and propane. Accordingly,naphtha generally has a boiling range between ambient temperature andabout 150 degrees F. (about 65 degrees C.), and the molecules boilingwithin this range will contain between about 5 and about 8 carbon atoms.The naphtha produced by this process will usually have a low octanerating due to the highly paraffinic nature of Fischer-Tropsch materials.Consequently, the naphtha produced by this process generally is notsuitable for use as a transportation fuel without further processing.However, the naphtha produced may be used as feed to an ethylene crackerwithout additional processing. Hydrocarbons having less than 5 carbonatoms in the molecule are normally gaseous at ambient temperature andare included among the overhead gases and may be recycled upstream inthe Fischer-Tropsch processing train before or after optionallyrecovering the LPG (C₃ and C₄) fraction.

[0015] Processing schemes similar to the process of the presentinvention have been proposed and been commercially practiced forconventional petroleum derived products. See, for example, U.S. Pat.Nos. 5,976,354; 5,980,729; 6,337,010 B1; and 6,432,297 B1. However noneof these processing schemes were intended for the processing ofFischer-Tropsch derived materials and their purpose is quite different.In addition, for most of these process schemes the primary product ofconcern is the lubricating base oil fraction. In the present process,while a lubricating base oil may be one of the products recovered, theprimary product of interest is the diesel fuel product. Accordingly, thetemperature conditions under which the separation between the dieselfraction and the base oil fraction is made is carefully controlled toassure that the portion of the isomerized base oil fraction which isblended back into the diesel fraction will produce a diesel producthaving the desired properties. In addition, since most of theseprocesses are concerned with processing petroleum derived feeds, thehydroprocessing operations to which the feed is subjected prior toseparation of the diesel and base oil fractions is for a differentpurpose, typically involving a hydrotreating operation to remove sulfurand nitrogen (see U.S. Pat. No. 5,976,354) or a hydrocracking operationto reduce the average molecular weight of the feed (see U.S. Pat. No.6,337,010 B1). In the present process, the hydroprocessing operation isprimarily intended to saturate the olefins and to remove the oxygenates.

[0016] As used in this disclosure the words “comprises” or “comprising”are intended as an open-ended transition meaning the inclusion of thenamed elements, but not necessarily excluding other unnamed elements.The phrases “consists essentially of” or “consisting essentially of” areintended to mean the exclusion of other elements of any essentialsignificance to the composition. The phrases “consisting of” or“consists of” are intended as a transition meaning the exclusion of allbut the recited elements with the exception of only minor traces ofimpurities.

SUMMARY OF THE INVENTION

[0017] The present invention is directed to a process for producing apremium Fischer-Tropsch diesel fuel which comprises (a) treating a waxyFischer-Tropsch feed recovered from a Fischer-Tropsch synthesis in ahydroprocessing zone under hydroprocessing conditions in the presence ofa hydroprocessing catalyst intended to saturate the olefins and toremove the oxygenates that are present in the feed, whereby a firstFischer-Tropsch intermediate product is produced with reduced olefinsand oxygenates relative to the Fischer-Tropsch feed; (b) separating thefirst Fischer-Tropsch intermediate product in a separation zone into aheavy Fischer-Tropsch fraction and a light Fischer-Tropsch fractionunder controlled separation conditions wherein the light Fischer-Tropschfraction is characterized by an end boiling point falling within theboiling range of diesel, and the heavy Fischer-Tropsch fraction beingcharacterized by a boiling range above that of the light Fischer-Tropschfraction; (c) contacting the heavy Fischer-Tropsch fraction with ahydroisomerization catalyst in a hydroisomerization zone underhydroisomerization conditions selected to improve the cold flowproperties of the heavy Fischer-Tropsch fraction and recovering anisomerized heavy Fischer-Tropsch fraction; (d) mixing the isomerizedheavy Fischer-Tropsch fraction with at least a portion of the lightFischer-Tropsch fraction of (b); and (e) recovering from the blend aFischer-Tropsch derived diesel product meeting a target value for atleast one pre-selected specification for diesel fuel. The heavyFischer-Tropsch fraction will generally have an initial boiling pointwithin the lower end of the boiling range for lubricating base oil andthe upper end of the boiling range for diesel, i.e., the initial boilingpoint will usually be between about 550 degrees F. (about 285 degreesC.) and about 750 degrees F. (about 400 degrees C.). However, in orderto meet the target value for the selected specification orspecifications for the diesel product, it may under certaincircumstances be desirable to produce more of the heavy fraction bylowering the initial boiling point of the heavy fraction below 600degrees F., perhaps as low as 450 degrees F. (about 230 degrees C.). Inthis instance, the amount of the heavy fraction that will be isomerizedand blended back into the diesel will be significantly increased.

[0018] The hydroprocessing conditions in the first step of the processused to saturate the olefins and remove the oxygenates present in theFischer-Tropsch feed are preferably mild and usually are selected tominimize the cracking of the molecules. However, by varying theconversion rate of the hydroprocessing operation, the amount of dieselor of lubricating base oil may be maximized. For example, by operatingat a higher conversion, typically greater than about 20 percentconversion, the amount of diesel produced by the process may beincreased, since a portion of the C₂₀ plus molecules present in the feedwill be cracked into products within the boiling range of transportationfuels. Similarly, by minimizing the amount of conversion in this step,generally less than 20 percent conversion and preferably 5 percentconversion or less, the amount of base oil produced will be maximizeddue to the very low cracking rate.

[0019] As used in this disclosure “conversion” of a hydrocarbonfeedstock refers to the percent of the hydrocarbons recovered from thehydroprocessing zone which have an initial boiling point above a givenreference temperature following the conversion of the Fischer-Tropschfeed into products boiling below the reference temperature. See U.S.Pat. No. 6,224,747. For the purposes of this disclosure the referencetemperature selected is usually about 650 degrees F. (340 degrees C.).

[0020] A portion of the isomerized heavy Fischer-Tropsch fractionproduced is blended back with the diesel in order to meet the targetvalue for one or more pre-selected specifications for diesel. Oneskilled in the art will recognize that the specification orspecifications selected will depend on the nature of the operation andthe market into which the diesel product is to be sold. Generally, thediesel specification or specifications selected will include one or moreof the cold filter plugging point, the cloud point, or the pour point.Each of these specifications may be readily controlled in the dieselproduct by the blending back a portion of the isomerized heavyFischer-Tropsch fraction.

[0021] In most embodiments of the invention, the separation zone willinclude at least two separation zones, referred to herein as a first anda second separation zone. The first separation zone, which in mostembodiments will comprise a hot high pressure separator, is used toseparate the heavy Fischer-Tropsch fraction from the naphtha, diesel andgaseous hydrogen rich fraction and usually will be operated at atemperature which is about 50 degrees F. (28 degrees C.) below thetemperature of the hydroprocessing zone. The second separation zone,which in most embodiments will comprise a cold high pressure separator,is used to separate the overhead gases from the remaining hydrocarbonsboiling in the range of transportation fuels. The operation of theseparation zone is critical to the invention, since the separationbetween the heavy and light Fischer-Tropsch fractions will determine howmuch of those hydrocarbons boiling in the diesel range will beisomerized along with the heavy fraction which is will be blended backas part of the final diesel product.

[0022] In order to facilitate the separation in the high pressureseparator it is preferable that a stripping gas be used. Strippinggases, such as, for example, steam or hydrogen may be employed in thehot high pressure separator. Generally hydrogen is preferred as thestripping gas in the present scheme.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a diagram illustrating a process scheme which representsone embodiment of the invention.

[0024]FIG. 2 illustrates an alternative embodiment of the process schemewhich incorporates a recycle loop to increase the production of liquidfuels.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention may be more clearly understood by referenceto FIG. 1 which represents one embodiment of the process scheme. In FIG.1 the Fischer-Tropsch condensate feed 2 and the Fischer-Tropsch waxyfeed 4 are shown separately prior to entering the hydrotreating reactor6 via a common conduit 8 where the feeds are also mixed with hydrogenfrom line 11 which is provided by make-up hydrogen entering by lines 9and 10 and by recycle hydrogen from line 28. In the hydrotreatingreactor 6 the olefins present in the feed are saturated and theoxygenates, mostly consisting of alcohols, are removed. The effluentfrom the hydrotreating reactor referred to in this disclosure as thefirst Fischer-Tropsch intermediate is carried via line 12 to the firstseparation zone 14 comprising a hot high pressure separator where theheavy Fischer-Tropsch fraction comprising primarily waxy materialboiling in the base oil range, but also including at least somehydrocarbons boiling in the diesel range, are separated from a lowerboiling Fischer-Tropsch fraction which includes hydrocarbons boilingboth in the range of naphtha and diesel as well as overhead gaseouscomprising hydrogen and C₄ minus hydrocarbons. The hot high pressureseparator is usually operated at a temperature that is at least 50degrees F. (28 degrees C.) below the operating temperature of thehydrotreating reactor 6. The heavy Fischer-Tropsch fraction is collectedin conduit 16 and carried to the hydroisomerization unit 18. Hydrogenfor the isomerization step is added from make-up hydrogen via lines 9and 19. Returning to the hot high pressure separator 14, the lowerboiling hydrocarbons and overhead gaseous are collected by conduit 20and carried to the second separation zone which comprises a cold highpressure separator 22. In the cold high pressure separator the hydrogenrich overhead gaseous are separated from those hydrocarbons boiling inthe range of transportation fuels. The hydrogen rich overhead gases passvia line 24 to an optional recycle gas scrubber 26 in order to removeany hydrogen sulfide or ammonia present prior to being sent via line 28to the recycle gas compressor 30 to be recycled by line 11 back to thehydrotreating reactor 6. The hydrocarbons comprising primarily thoseboiling within the range of naphtha and diesel are recovered by line 32from the cold high pressure separator and sent to a low pressureseparator 34.

[0026] Returning to the hydroisomerization unit 18, the heavyFischer-Tropsch fraction which contains most of the Fischer-Tropsch waxis isomerized to increase the isoparaffin content of the fraction andimprove its cold flow properties, such as the cold filter pluggingpoint, the pour point, and the VI, as well as the cloud point. Theisomerized heavy Fischer-Tropsch fraction is collected in line 36 andpassed to the hydrofinishing reactor 38 where the oxidation stability isfurther improved. The isomerized and hydrofinished heavy fraction iscarried by line 40 to a high pressure separator 42 where the hydrogenrich overhead gases are collected and carried by line 44 back to thecold high pressure separator 22 to be recycled to the hydrotreatingunit. The effluent from cold high pressure separator containing theheavy fraction is carried by line 46 to the low pressure separator 34where the isomerized and hydrofinished heavy fraction are mixed with thelight fraction coming from the cold high pressure separator 22. Theoverhead gases comprising primarily C₄ minus hydrocarbons are collectedfrom the top of the low pressure separator by line 47 and carried to thetop of a product stripper 48. The mixture of heavy and lightFischer-Tropsch fractions are collected in line 49 from the bottom ofthe low pressure separator and passed to the lower section of theproduct stripper 48 where additional C₄ minus hydrocarbons are separatedfrom the C₅ plus hydrocarbons. The C4 minus hydrocarbons are collectedfrom stripper by conduit 50. The product stream comprising C₅ plushydrocarbons are collected in line 52 and passed to the atmosphericdistillation unit 54 where the naphtha 56 and diesel 58 are collectedseparately from any remaining C₄ minus hydrocarbons in line 60. Theheavy bottoms fraction is collected and sent via line 62 to the vacuumdistillation unit 64 where the light base oil fraction 66, medium baseoil fraction 68, and heavy base oil fraction 70 are shown beingseparately collected.

[0027] By controlling the operation of the hot high pressure separator14, the non-waxy molecules are removed from the feed to thehydroisomerization unit 18 and prevented from contacting theisomerization catalyst. The light Fischer-Tropsch fraction comprisingthe majority of the diesel and substantially all of the naphtha fractionthus bypass the isomerization operation making the isomerization stepmuch more efficient, since it handles a smaller volume of hydrocarbonsthan it might otherwise. Only that fraction containing the majority ofthe Fischer-Tropsch wax will enter the hydroisomerization zone. Thisseparation step also is used to meet the specifications for the dieselfuel that is produced by the integrated process. By blending a portionof the isomerized and hydrofinished heavy Fischer-Tropsch fraction withthe diesel, the overall cold flow properties and cloud point of thediesel product is improved without the necessity of hydroisomerizing andhydrofinishing the entire diesel product. Most of the heavy fractionwhich is recovered with the diesel product from the atmosphericdistillation column 54 will comprise a lighter base oil fraction, i.e.,the base oil fraction which has an upper boiling point of less than 750degrees F. (400 degrees C.). Thus by controlling the cut points in thehot high pressure separator and in the fractionation operation theamount of isomerized and hydrofinished base oil blended into the dieselproduct may be controlled. In addition, the operation of thehydroisomerization unit may be controlled to optimize the conversion ofthe heavy fraction which also will contribute to the properties of thefinal diesel product recovered from the operation.

[0028] As already noted, the operation of the hydroprocessing unit,shown in the drawing as the hydrotreating unit 6, may be varied to makemore hydrocarbons boiling in the range of transportation fuels. Byoperating under more sever conditions to increase the conversion, thelarger molecules may be cracked to yield more diesel.

[0029]FIG. 2 illustrates an alternative embodiment of the inventionwhich contains a recycle loop 17 which recycles the heavyFischer-Tropsch fraction from conduit 16 back to Fischer-Tropsch waxyfeed 4. This embodiment is able to maximize the production of liquidfuels by operating the hydroprocessing reactor 6 at a higher conversion,preferably above 20 percent conversion, and by recycling the heavyhydrocarbons collected in the first separation zone back to thehydroprocessing reactor 6 by means of recycle loop 17 for furtherconversion. In this embodiment the amount of base oil is minimized andthe amount of liquid fuels is maximized.

[0030] As an integrated process, the process of the present inventionalso allows for the efficient recycling of the hydrogen rich C₄ minusoverhead gases to the hydroprocessing zone, the catalytic dewaxing zone,and the hydrofinishing zone. It is generally advantageous to operate thehydroprocessing reactor, catalytic dewaxing reactor, and hydrofinishingreactor at substantially the same pressure, since such operation reducesthe capital cost by saving on the need for additional pumps andcompressors. However, hydroisomerization generally has an optimalreaction pressure below that for hydrocracking, hydrotreating, andhydrofinishing. Therefore, it may be advantageous under certaincircumstances to operate the catalytic dewaxing unit at a lower pressurethan the hydroprocessing unit and the hydrofinishing unit. See forexample, U.S. Pat. No. 6,337,010 B1.

[0031] Fischer-Tropsch Synthesis

[0032] In the Fischer-Tropsch synthesis process, liquid and gaseoushydrocarbons are formed by contacting a synthesis gas (syngas)comprising a mixture of hydrogen and carbon monoxide with aFischer-Tropsch catalyst under suitable temperature and pressurereactive conditions. The Fischer-Tropsch reaction is typically conductedat temperatures of from about 300 degrees F. to about 700 degrees F.(about 150 degrees C. to about 370 degrees C.) preferably from about 400degrees F. to about 550 degrees F. (about 205 degrees C. to about 230degrees C.); pressures of from about 10 psia to about 600 psia (0.7 barsto 41 bars), preferably 30 psia to 300 psia (2 bars to 21 bars), andcatalyst space velocities of from about 100 cc/g/hr. to about 10,000cc/g/hr., preferably 300 cc/g/hr. to 3,000 cc/g/hr.

[0033] The products may range from C_(1 to C) ₂₀₀ plus hydrocarbons witha majority, by weight, in the C₅-C₁₀₀ plus range. The reaction can beconducted in a variety of reactor types, for example, fixed bed reactorscontaining one or more catalyst beds, slurry reactors, fluidized bedreactors, or a combination of different type reactors. Such reactionprocesses and reactors are well known and documented in the literature.Slurry Fischer-Tropsch processes, which is a preferred process forproducing the feed stocks used for carrying out the invention, utilizesuperior heat (and mass) transfer characteristics for the stronglyexothermic synthesis reaction and are able to produce relatively highmolecular weight, paraffinic hydrocarbons when using a cobalt catalyst.In a slurry process, a syngas comprising a mixture of hydrogen andcarbon monoxide is bubbled up in the reactor as a third phase through aslurry which comprises a particulate Fischer-Tropsch type hydrocarbonsynthesis catalyst dispersed and suspended in a slurry liquid comprisinghydrocarbon products of the synthesis reaction which are liquid at thereaction conditions. The mole ratio of the hydrogen to the carbonmonoxide may broadly range from about 0.5 to about 4, but is moretypically within the range of from about 0.7 to about 2.75 andpreferably from about 0.7 to about 2.5. A particularly preferredFischer-Tropsch process is taught in EP 0609079, also completelyincorporated herein by reference for all purposes.

[0034] Suitable Fischer-Tropsch catalysts comprise one or more Group VIIcatalytic metals such as Fe, Ni, Co, Ru and Re, with cobalt generallybeing one preferred embodiment. Additionally, a suitable catalyst maycontain a promoter. Thus, in one embodiment, the Fischer-Tropschcatalyst will comprise effective amounts of cobalt and one or more ofRe, Ru, Pt, Fe, Ni, Th, Zr, Hf, U, Mg and La on a suitable inorganicsupport material, preferably one which comprises one or more refractorymetal oxides. In general, the amount of cobalt present in the catalystis between about 1 and about 50 weight percent of the total catalystcomposition. The catalysts can also contain basic oxide promoters suchas ThO₂, La₂O3, MgO, K₂O and TiO₂, promoters such as ZrO₂, noble metals(Pt, Pd, Ru, Rh, Os, Ir), coinage metals (Cu, Ag, Au), and othertransition metals such as Fe, Mn, Ni, and Re. Suitable support materialsinclude alumina, silica, magnesia and titania or mixtures thereof.Preferred supports for cobalt containing catalysts comprise alumina ortitania. Useful catalysts and their preparation are known andillustrated in U.S. Pat. No. 4,568,663, which is intended to beillustrative but non-limiting relative to catalyst selection.

[0035] The products from the Fischer-Tropsch process usually arecollected separately as a waxy fraction which contains the majority ofthe Fischer-Tropsch wax, a condensate fraction which contains thehydrocarbons boiling in the range of transportation fuels, and a gaseousfraction containing unreacted hydrogen and carbon monoxide and C₄ minushydrocarbons. The waxy fraction is normally a solid at ambienttemperature and represents the fraction which makes up the majority ofthe material that will be isomerized in the present process. Thecondensate fraction, in addition to containing most of the hydrocarbonsboiling in the range of naphtha and diesel, also contains oxygenates,mostly in form of alcohols, which must be removed prior to furtherprocessing. All of the fractions contain a significant amount of olefinswhich must be saturated in the hydroprocessing step.

[0036] Hydroprocessing

[0037] Hydroprocessing in the present invention refers to the stepintended primarily for the purpose of removing any residual nitrogen,saturating the olefins, and removing oxygenates that may be present inthe Fischer-Tropsch feed stock. By increasing the severity of thehydroprocessing step, the amount of diesel recovered in the finalproduct slate may be increased. For the purposes of this discussion, theterm hydroprocessing is intended to refer to either hydrotreating orhydrocracking. Hydroisomerization and hydrofinishing, while also a typeof hydroprocessing, will be treated separately because of theirdifferent functions in the process scheme.

[0038] Hydrotreating refers to a catalytic process, usually carried outin the presence of free hydrogen, in which the primary purpose when usedto process conventional petroleum derived feed stocks is the removal ofvarious metal contaminants, such as arsenic; heteroatoms, such as sulfurand nitrogen; and aromatics from the feed stock. In the present process,the primary purpose is to saturate the olefins and remove the oxygenatesin the feed stock prior to the catalytic dewaxing operation. Generally,in hydrotreating operations cracking of the hydrocarbon molecules, i.e.,breaking the larger hydrocarbon molecules into smaller hydrocarbonmolecules is minimized. For the purpose of this discussion the termhydrotreating refers to a hydroprocessing operation in which theconversion is 20 percent or less.

[0039] Hydrocracking refers to a catalytic process, usually carried outin the presence of free hydrogen, in which the cracking of the largerhydrocarbon molecules is the primary purpose of the operation. Incontrast to hydrotreating, the conversion rate for hydrocracking, forthe purpose of this disclosure. shall be more than 20 percent.Hydrogenation of the olefins and removal of the oxygenates as well asdenitrification of the feedstock also will occur. In the presentinvention, cracking of the hydrocarbon molecules may be desirable inorder to increase the yield of diesel and minimize the amount of heavyFischer-Tropsch fraction passing through the catalytic dewaxingoperation.

[0040] Catalysts used in carrying out hydrotreating and hydrocrackingoperations are well known in the art. See for example U.S. Pat. Nos.4,347,121 and 4,810,357, the contents of which are hereby incorporatedby reference in their entirety, for general descriptions ofhydrotreating, hydrocracking, and of typical catalysts used in each ofthe processes. Suitable catalysts include noble metals from Group VIIIA(according to the 1975 rules of the International Union of Pure andApplied Chemistry), such as platinum or palladium on an alumina orsiliceous matrix, and unsulfided Group VIIIA and Group VIB, such asnickel-molybdenum or nickel-tin on an alumina or siliceous matrix. U.S.Pat. No. 3,852,207 describes a suitable noble metal catalyst and mildconditions. Other suitable catalysts are described, for example, in U.S.Pat. Nos. 4,157,294 and 3,904,513. The non-noble hydrogenation metals,such as nickel-molybdenum, are usually present in the final catalystcomposition as oxides, or more preferably or possibly, as sulfides whensuch compounds are readily formed from the particular metal involved.Preferred non-noble metal catalyst compositions contain in excess ofabout 5 weight percent, preferably about 5 to about 40 weight percentmolybdenum and/or tungsten, and at least about 0.5, and generally about1 to about 15 weight percent of nickel and/or cobalt determined as thecorresponding oxides. Catalysts containing noble metals, such asplatinum, contain in excess of 0.01 percent metal, preferably between0.1 and 1.0 percent metal. Combinations of noble metals may also beused, such as mixtures of platinum and palladium.

[0041] The hydrogenation components can be incorporated into the overallcatalyst composition by any one of numerous procedures. Thehydrogenation components can be added to matrix component by co-mulling,impregnation, or ion exchange and the Group VI components, i.e.;molybdenum and tungsten can be combined with the refractory oxide byimpregnation, co-mulling or co-precipitation. Although these componentscan be combined with the catalyst matrix as the sulfides, that isgenerally not preferred, as the sulfur compounds can interfere with theFischer-Tropsch catalysts.

[0042] The matrix component can be of many types including some thathave acidic catalytic activity. Ones that have activity includeamorphous silica-alumina or may be a zeolitic or non-zeoliticcrystalline molecular sieve. Examples of suitable matrix molecularsieves include zeolite Y, zeolite X and the so called ultra stablezeolite Y and high structural silica:alumina ratio zeolite Y such asthat described in U.S. Pat. Nos. 4,401,556; 4,820,402; and 5,059,567.Small crystal size zeolite Y, such as that described in U.S. Pat. No.5,073,530 can also be used. Non-zeolitic molecular sieves which can beused include, for example, silicoaluminophosphates (SAPO),ferroaluminophosphate, titanium aluminophosphate and the various ELAPOmolecular sieves described in U.S. Pat. No. 4,913,799 and the referencescited therein. Details regarding the preparation of various non-zeolitemolecular sieves can be found in U.S. Pat. Nos. 5,114,563 (SAPO) and4,913,799 and the various references cited in U.S. Pat. No. 4,913,799.Mesoporous molecular sieves can also be used, for example the M41 Sfamily of materials as described in J. Am. Chem. Soc.,114:10834-10843(1992)), MCM-41; U.S. Pat. Nos. 5,246,689; 5,198,203; and5,334,368; and MCM-48 (Kresge et al., Nature 359:710 (1992)). Suitablematrix materials may also include synthetic or natural substances aswell as inorganic materials such as clay, silica and/or metal oxidessuch as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-berylia, silica-titania as well as ternary compositions, such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia,and silica-magnesia zirconia. The latter may be either naturallyoccurring or in the form of gelatinous precipitates or gels includingmixtures of silica and metal oxides. Naturally occurring clays which canbe composited with the catalyst include those of the montmorillonite andkaolin families. These clays can be used in the raw state as originallymined or initially subjected to calumniation, acid treatment or chemicalmodification.

[0043] In performing the hydrocracking and/or hydrotreating operation,more than one catalyst type may be used in the reactor. The differentcatalyst types can be separated into layers or mixed.

[0044] Hydrocracking conditions have been well documented in theliterature. In general, the overall LHSV is about 0.1 hr⁻¹ to about 15.0hr⁻¹ (v/v), preferably from about 0.25 hr⁻¹ to about 2.5 hr⁻¹. Thereaction pressure generally ranges from about 500 psig to about 3500psig (about 10.4 MPa to about 24.2 MPa, preferably from about 1500 psigto about 5000 psig (about 3.5 MPa to about 34.5 MPa). Hydrogenconsumption is typically from about 500 to about 2500 SCF per barrel offeed (89.1 to 445 m3H2/m3 feed). Temperatures in the reactor will rangefrom about 400 degrees F. to about 950 degrees F. (about 205 degrees C.to about 510 degrees C.), preferably ranging from about 650 degrees F.to about 850 degrees F. (about 340 degrees C. to about 455 degrees C.).

[0045] Typical hydrotreating conditions vary over a wide range. Ingeneral, the overall LHSV is about 0.5 to 5.0. The total pressureranging from about 200 psig to about 2000 psig. Hydrogen recirculationrates are typically greater than 50 SCF/Bbl, and are preferably between1000 and 5000 SCF/Bbl. Temperatures in the reactor will range from about400 degrees F. to about 800 degrees F. (about 205 degrees C. to about425 degrees C.).

[0046] In order to achieve maximum flexibility in the process, thehydroprocessing reactor may be filled with a hydroprocessing catalystthat, when operated at low temperatures, only provides hydrotreating ofthe feedstock, therefore allowing for the maximum production of highquality lubricating base oils. When the hydroprocessing zone is operatedat a relatively higher temperature, higher conversion of theFischer-Tropsch wax feed to smaller, lower boiling molecules isaccomplished. The catalyst selected for this varying operation can be adual-function (hydrogenation/acidic conversion) hydrocracking catalystwhich will provide hydrotreating when maximum lubricating base oilproduction is desired and hydrotreating plus hydrocracking when maximumfuel production is the goal.

[0047] Separation Zone

[0048] In the process of the present invention, the separation zone isused to separate those hydrocarbons boiling in the range oftransportation fuels, i.e., in range of naphtha and diesel (referred toas the light Fischer-Tropsch fraction) from those hydrocarbons boilingin the base oil range (referred to as the heavy Fischer-Tropschfraction) from the first Fischer-Tropsch intermediate product collectedfrom the hydroprocessing operation. Generally, the cut-point for theseparation between the heavy Fischer-Tropsch fraction and the lightFischer-Tropsch fraction will be within the temperature range of betweenabout 550 degrees F. and about 750 degrees F. (about 285 degrees C. toabout 400 degrees C.). Usually the cut-point will be about 600 degreesF. (315 degrees C.). However, due to the unique properties ofFischer-Tropsch derived products the cut-point may be as low as 450degrees F. (about 230 degrees C.). The precise cut-point selected willdepend upon how much of the base oil present in the firstFischer-Tropsch intermediate product is selected for isomerization. Theselection of how much base oil to send to the catalytic dewaxing zonewill depend upon the target value selected for the property orproperties of the final diesel product. In general, the lower thecut-point between the heavy and light fractions, the moreFischer-Tropsch wax will be sent to the catalytic dewaxing zone. Morewax isomerization will result in improved cold-flow properties in thediesel product. However, most of the Fischer-Tropsch wax is concentratedin the higher boiling fractions. Thus dropping the cut-point below acertain temperature yields decreasing benefits in the properties of thediesel product. In addition, the more heavy Fischer-Tropsch fractionsent to the catalytic dewaxing zone, the larger the reaction vessel mustbe to handle the increased volume of material which results in highercapital costs. Thus one skilled in the art will recognize that a balancemust be achieved between the size of the catalytic dewaxing reactor andthe properties of the final diesel product. The diesel product must meetthe target values for the selected specification while at the same timeminimizing the amount of material sent to the catalytic dewaxing unit.

[0049] The separation in the separation zone will usually take place ata temperature that is at least 50 degrees F. (30 degrees C.) below theoperating temperature of the hydroprocessing reactor. This is necessaryin the present scheme due to the nature of the Fischer-Tropsch feed.This aspect differs from the operation of similar schemes described inthe prior art which are directed to the processing of conventionalpetroleum derived feed stocks. See U.S. Pat. Nos. 5,976,354 and6,432,297. Although the configuration of the equipment used in the priorart schemes is similar to that used for the scheme described herein, theactual operation is quite different. In processing conventionalpetroleum feeds, the separator is operated at substantially the sametemperature as the hydroprocessing operation. Since petroleum derivedfractions which include diesel are not waxy, substantially all of thediesel is recovered along with the naphtha and overhead gases in theprior art processes. Virtually none of the final diesel product haspassed through the catalytic dewaxing unit in these schemes. In thepresent process, due to the waxy nature of the Fischer-Tropsch diesel, asignificant amount of the material that will be included in the finaldiesel product is isomerized. Typically, between about 25 and about 75volume percent of the final diesel product will have passed through thecatalytic dewaxing unit. The actual amount of the final diesel productwhich has passed through the catalytic dewaxing unit will depend on thetarget value selected for the diesel specification.

[0050] Usually the separation zone will comprise at least two separationvessels. In the drawing, the separation zone comprises a hot highpressure separator and a cold high pressure separator. In this schemethe hot high pressure separator makes the initial separation between theheavy Fischer-Tropsch fraction and the light Fischer-Tropsch fraction.While this separation will take place at a relatively high temperature,it usually will still be at a temperature that is at least 50 degrees F.(30 degrees C.) lower than the temperature in the hydroprocessingreactor. In the cold high pressure separator, the overhead gases areseparated from the hydrocarbons boiling in the range of thosetransportation fuels which will not pass through the catalytic dewaxingzone.

[0051] Catalytic Dewaxing and Hydroisomerization

[0052] Catalytic dewaxing consists of three main classes, conventionalhydrodewaxing, complete hydroisomerization dewaxing, and partialhydroisomerization dewaxing. All three classes involve passing a mixtureof a waxy hydrocarbon stream and hydrogen over a catalyst that containsan acidic component to reduce the normal and slightly branchediso-paraffins in the feed and increase the proportion of other non-waxyspecies. The method selected for dewaxing a feed typically depends onthe product quality, and the wax content of the feed, with conventionalhydrodewaxing often preferred for low wax content feeds. The method fordewaxing can be effected by the choice of the catalyst. The generalsubject is reviewed by Avilino Sequeira, in Lubricant Base Stock and WaxProcessing, Marcel Dekker, Inc., pages 194-223. The determinationbetween conventional hydrodewaxing, complete hydroisomerizationdewaxing, and partial hydroisomerization dewaxing can be made by usingthe n-hexadecane isomerization test as described in U.S. Pat. No.5,282,958. When measured at 96 percent, n-hexadecane conversion usingconventional hydrodewaxing catalysts will exhibit a selectivity toisomerized hexadecanes of less than 10 percent, partialhydroisomerization dewaxing catalysts will exhibit a selectivity toisomerized hexadecanes of greater than 10 percent to less than 40percent, and complete hydroisomerization dewaxing catalysts will exhibita selectivity to isomerized hexadecanes of greater than or equal to 40percent, preferably greater than 60 percent, and most preferably greaterthan 80 percent.

[0053] In conventional hydrodewaxing, the pour point is lowered byselectively cracking the wax molecules mostly to smaller paraffins usinga conventional hydrodewaxing catalyst, such as, for example ZSM-5.Metals may be added to the catalyst, primarily to reduce fouling. In thepresent invention conventional hydrodewaxing may be used to increase theyield of diesel in the final product slate by cracking theFischer-Tropsch wax molecules. In the present process, the isomerizationof the paraffins also is used to improve the cold flow properties andcloud point of the diesel fraction. Typical conditions forhydroisomerization as used in the present process involve temperaturesfrom about 400 degrees F. to about 800 degrees F. (about 200 degrees C.to about 425 degrees C.), pressures from about 100 psig to 2000 psig,and space velocities from about 0.2 to 5 hr⁻¹.

[0054] Complete hydroisomerization dewaxing typically achieves highconversion levels of wax by isomerization to non-waxy iso-paraffinswhile at the same time minimizing the conversion by cracking. Since waxconversion can be complete, or at least very high, this processtypically does not need to be combined with additional dewaxingprocesses to produce a lubricating oil base stock with an acceptablepour point. Complete hydroisomerization dewaxing uses a dual-functionalcatalyst consisting of an acidic component and an active metal componenthaving hydrogenation activity. Both components are required to conductthe isomerization reaction. The acidic component of the catalysts usedin complete hydroisomerization preferably include an intermediate poreSAPO, such as SAPO-11, SAPO-31, and SAPO-41, with SAPO-11 beingparticularly preferred. Intermediate pore zeolites, such as ZSM-22,ZSM-23, SSZ-32, ZSM-35, and ZSM48, also may be used in carrying outcomplete hydroisomerization dewaxing. Typical active metals includemolybdenum, nickel, vanadium, cobalt, tungsten, zinc, platinum, andpalladium. The metals platinum and palladium are especially preferred asthe active metals, with platinum most commonly used.

[0055] In partial hydroisomerization dewaxing, a portion of the wax isisomerized to iso-paraffins using catalysts that can isomerize paraffinsselectively, but only if the conversion of wax is kept to relatively lowvalues (typically below 50 percent). At higher conversions, waxconversion by cracking becomes significant, and yield losses oflubricating base stock becomes uneconomical. Like completehydroisomerization dewaxing, the catalysts used in partialhydroisomerization dewaxing include both an acidic component and ahydrogenation component. The acidic catalyst components useful forpartial hydroisomerization dewaxing include amorphous silica aluminas,fluorided alumina, and 12-ring zeolites (such as Beta, Y zeolite, Lzeolite). The hydrogenation component of the catalyst is the same asalready discussed with complete hydroisomerization dewaxing. Because thewax conversion is incomplete, partial hydroisomerization dewaxing mustbe supplemented with an additional dewaxing technique, typically solventdewaxing, complete hydroisomerization dewaxing, or conventionalhydrodewaxing in order to produce a lubricating base stock with anacceptable pour point (below about +10 degrees F. or −12 degrees C.).

[0056] In preparing those catalysts containing a non-zeolitic molecularsieve and having a hydrogenation component for use in the presentinvention, it is usually preferred that the metal be deposited on thecatalyst using a non-aqueous method. Catalysts, particularly catalystscontaining SAPO's, on which the metal has been deposited using thenon-aqueous method, have shown greater selectivity and activity thanthose catalysts which have used an aqueous method to deposit the activemetal. The non-aqueous deposition of active metals on non-zeoliticmolecular sieves is taught in U.S. Pat. No. 5,939,349. In general, theprocess involves dissolving a compound of the active metal in anon-aqueous, non-reactive solvent and depositing it on the molecularsieve by ion exchange or impregnation.

[0057] Hydrofinishing

[0058] Hydrofinishing operations are intended to improve the UVstability and color of the products. It is believed this is accomplishedby saturating the double bonds present in the hydrocarbon molecules,including those found in aromatics, especially polycyclic aromatics. Asshown in the drawing, only the heavy Fischer-Tropsch fraction which haspassed through the catalytic dewaxer is sent to a hydrofinisher. Ageneral description of the hydrofinishing process may be found in U.S.Pat. Nos. 3,852,207 and 4,673,487. As used in this disclosure the termUV stability refers to the stability of the lubricating base oil orother products when exposed to ultraviolet light and oxygen. Instabilityis indicated when a visible precipitate forms or darker color developsupon exposure to ultraviolet light and air which results in a cloudinessor floc in the product. It may also be desirable that the diesel productprepared by the process of the present invention be UV stabilized priorto marketing in which case this fraction may also be hydrofinished.

[0059] Typically, the total pressure in the hydrofinishing zone will bebetween about 200 psig and about 3000 psig, with pressures in the rangeof about 500 psig and about 2000 psig being preferred. Temperatureranges in the hydrofinishing zone are usually in the range of from about400 degrees F. (about 205 degrees C.) to about 650 degrees F. (about 345degrees C.). The LHSV is usually within the range of from about 0.3 toabout 5.0. Hydrogen is usually supplied to the hydrofinishing zone at arate of from about 1000 to about 10,000 SCF per barrel of feed.Typically the hydrogen is fed at a rate of about 3000 SCF per barrel offeed.

[0060] Suitable hydrofinishing catalysts typically contain a Group VIIImetal component together with an oxide support. Metals or compounds ofthe following metals are useful in hydrofinishing catalysts includenickel, ruthenium, rhodium, iridium, palladium, platinum, and osmium.Preferably the metal or metals will be platinum, palladium or mixturesof platinum and palladium. The refractory oxide support usually consistsof alumina, silica, silica-alumina, silica-alumina-zirconia, and thelike. The catalyst may optionally contain a zeolite component. Typicalhydrofinishing catalysts are disclosed in U.S. Pat. Nos. 3,852,207;4,157,294; and 4,673,487.

[0061] Diesel Product

[0062] In the present invention the final diesel product is prepared byblending a lower boiling fraction of the isomerized heavy fraction backinto the diesel fraction recovered from the separation zone. Asillustrated in the drawing the isomerized heavy fraction and the lightfraction are blended together in the low pressure separator. The dieselproduct, including part of the isomerized heavy fraction, is shown inthe drawing as being separated from the lighter naphtha, C₄ minusfraction, and base oil in the atmospheric fractionation unit. Thevarious lube fractions may be further separated, if desired in a vacuumfractionation column.

[0063] In the present invention, the properties of diesel product may becontrolled at several points in the process. The first control point andthe most important are in the separation zone. As already noted, theseparation zone controls how much of the waxy material which will beincluded in the diesel product will pass though the hydroisomerizationoperation. The second point of control resides in the hydroisomerizationunit. By controlling the wax conversion, the cold flow properties of thediesel also may be adjusted. Finally, the properties of the dieselproduct may be controlled in the fractionation step. How much of theisomerized base oil fraction remains as part of the diesel product alsowill help determine what the final properties of the diesel product willbe. One skilled in the art will recognize that there are other schemesthan the one shown in the drawing to accomplish the overall processwithout departing from the spirit of the invention.

[0064] In the present invention the diesel fraction and isomerized baseoil fraction are blended to achieve a target value for at least onediesel specification. The diesel specifications will usually be selectedfrom one or more of the cold filter plugging point, the cloud point, andthe pour point. In the case of the cold filter plugging point, thetarget value will usually be a temperature of −10 degrees C. or less,preferably −20 degrees C. or less. The target value for cloud point willusually be a temperature of −8 degrees C. or less, preferably −18degrees C. or less. The target value for pour point will typically be−15 degrees C. or less, preferably −25 degrees C. or less.

[0065] The cold filter plugging point (“CFPP”) is a standard test usedto determine the ease with which fuel moves under suction through afilter grade representative of field equipment. The determination isrepeated periodically during steady cooling of the fuel sample, thelowest temperature at which the minimum acceptable level offilterability is still achieved being recorded as the “CFPP” temperatureof the sample. The details of the CFPP test and cooling regime arespecified in ASTM D-6371.

[0066] Pour point is the temperature at which a sample of the dieselfuel will begin to flow under carefully controlled conditions. In thisdisclosure, pour point, unless stated otherwise, is determined by thestandard analytical method ASTM D-5950.

[0067] Fischer-Tropsch Derived Lubricating Base Oil

[0068] In addition, to producing a premium diesel product, the presentinvention may also be used to produce a premium Fischer-Tropsch derivedlubricating base oil. Fischer-Tropsch derived base oils recovered fromthe process of this invention typically will contain very low sulfur andaromatics, have excellent oxidation stability, and excellent cold flowproperties. Generally, the lubricating base oils recovered from theprocess will have a kinematic viscosity of at least 3 cSt at 100 degreesC., preferably at least 4 cSt.; a pour point below 20 degrees C.,preferably below −12 degrees C.; and a VI that is usually greater than90, preferably greater than 100. The lower boiling base oils usuallywill be included in the final diesel blend, therefore, there is verylittle of the low viscosity material recovered from the vacuumdistillation column.

What we claim is:
 1. A process for producing a premium Fischer-Tropschdiesel fuel which comprises: (a) treating a waxy Fischer-Tropsch feedrecovered from a Fischer-Tropsch synthesis in a hydroprocessing zoneunder hydroprocessing conditions in the presence of a hydroprocessingcatalyst intended to saturate the olefins and to remove the oxygenatesthat are present in the feed, whereby a first Fischer-Tropschintermediate product is produced with reduced olefins and oxygenatesrelative to the Fischer-Tropsch feed; (b) separating the firstFischer-Tropsch intermediate product in a separation zone into a heavyFischer-Tropsch fraction and a light Fischer-Tropsch fraction undercontrolled separation conditions wherein the light Fischer-Tropschfraction is characterized by an end boiling point falling within theboiling range of diesel, and the heavy Fischer-Tropsch fraction beingcharacterized by a boiling range above that of the light Fischer-Tropschfraction; (c) contacting the heavy Fischer-Tropsch fraction with anhydroisomerization catalyst in a hydroisomerization zone underhydroisomerization conditions selected to improve the cold flowproperties of the heavy Fischer-Tropsch fraction and recovering anisomerized heavy Fischer-Tropsch fraction; (d) mixing the isomerizedheavy Fischer-Tropsch fraction with at least a portion of the lightFischer-Tropsch fraction of (b); and (e) recovering from the blend aFischer-Tropsch derived diesel product meeting a target value for atleast one pre-selected specification for diesel fuel.
 2. The process ofclaim 1 wherein the hydroprocessing catalyst in the hydroprocessing zonemay operate as either as a hydrotreating catalyst or as a hydrocrackingcatalyst depending on the hydroprocessing conditions selected.
 3. Theprocess of claim 1 wherein the conversion of the Fischer-Tropsch feed inthe hydroprocessing zone is greater than 20 percent.
 4. The process ofclaim 3 including the step of recycling part of the heavyFischer-Tropsch fraction recovered in the separation zone back to thehydroprocessing zone.
 5. The process of claim 1 wherein the conversionof the Fischer-Tropsch feed in the hydroprocessing zone is 20 percent orless.
 6. The process of claim 5 wherein the conversion of theFischer-Tropsch feed in the hydroprocessing zone is 5 percent or less.7. The process of claim 5 wherein the hydroprocessing conditions includea hydrogen partial pressure of between about 200 psig to about 2000psig, a temperature in the range of from about 400 degrees F. to about800 degrees F., a LHSV of between about 0.5 and about 5.0.
 8. Theprocess of claim 5 wherein the hydroprocessing catalyst comprises atleast one active metal selected from Group VIIIA of the Periodic Tableof the Elements and at least one active metal selected from Group VIB ofthe Periodic Table of the Elements, said active metals being present ona refractory support.
 9. The process of claim 5 further including theintermediate step of hydrofinishing the isomerized heavy Fischer-Tropschfraction of step (c) in a hydrofinishing zone under hydrofinishingconditions prior to blending the first portion of the isomerized heavyFischer-Tropsch fraction with the light Fischer-Tropsch fraction. 10.The process of claim 9 wherein the hydrofinishing conditions include apressure between about 500 psig and about 2000 psig, a temperature ofbetween about 400 degrees F. and about 650 degrees F., a LHSV betweenabout 0.3 and about 5.0.
 11. The process of claim 9 wherein a secondportion of the hydrofinished and isomerized heavy Fischer-Tropschfraction is also recovered separately as a lubricating base oil.
 12. Theprocess of claim 9 wherein the pressure in the hydroprocessing zone andin the hydrofinishing zone are substantially the same.
 13. The processof claim 12 wherein the hydoisomerization zone is operated at a lowerpressure than the hydroprocessing zone and the hydrofinishing zone. 14.The process of claim 1 wherein the light Fischer-Tropsch fraction has anend point falling within the range between about 450 degrees F. andabout 750 degrees F.
 15. The process of claim 1 wherein theisomerization catalyst in the catalytic dewaxing zone is ahydroisomerization catalyst.
 16. The process of claim 15 wherein thehydroisomerization catalyst contains a molecular sieve selected from thegroup consisting essentially of ZSM-22, ZSM-23, SSZ-32, ZSM-35, ZSM48,SAPO-11, SAPO-31, and SAPO-41.
 17. The process of claim 16 wherein thehydroisomerization catalyst contains an active metal selected fromplatinum, palladium, or a combination of platinum and palladium.
 19. Theprocess of claim 1 wherein the hydroisomerization conditions include atemperature of between about 400 degrees F. and about 800 degrees F., apressure of from about 100 psig to about 2000 psig, and an liquid hourlyspace velocity of between about 0.2 hr¹ and about 5 hr¹.
 20. The processof claim 1 wherein the pre-selected specification for diesel fuel towhich the Fischer-Tropsch diesel product is blended is the cold filterplugging point.
 21. The process of claim 20 wherein the target value forthe cold filter plugging point is 0 degrees C. or less.
 22. The processof claim 21 wherein the target value for the cold filter plugging pointis −20 degrees C. or less.
 23. The process of claim 1 wherein thepre-selected specification for diesel fuel to which the Fischer-Tropschdiesel product is blended is cloud point.
 24. The process of claim 23wherein the target value for the cloud point is about −5 degrees C. orless.
 25. The process of claim 24 wherein the target value for the cloudpoint is about −18 degrees C. or less.
 26. The process of claim 1wherein the pre-selected specification for diesel fuel to which theFischer-Tropsch diesel product is blended is pour point.
 27. The processof claim 26 wherein the target value for the pour point is about −15degrees C. or less.
 28. The process of claim 1 in which the separationzone of step (b) is divided into at least a first intermediateseparation zone and a second intermediate separation zone and whereinthe separation of step (b) includes the additional steps of (i)separately recovering from the first intermediate separation zone theheavy Fischer-Tropsch fraction and a mixture containing the lightFischer-Tropsch fraction and a hydrogen-rich C₄ minus fraction; (ii)feeding the mixture containing the light Fischer-Tropsch fraction andthe hydrogen-rich C₄ minus fraction to the second intermediateseparation zone; and (iii) recovering separately from the secondintermediate separation zone the light Fischer-Tropsch fraction and thehydrogen-rich C₄ minus fraction.
 29. The process of claim 28 wherein thehydrogen-rich C₄ minus fraction is recycled to the hydroprocessing zone.30. The process of claim 28 wherein the hydrogen-rich C₄ minus fractionis sent to the hydroisomerization zone.
 31. The process of claim 28wherein the hydrogen-rich C₄ minus fraction is sent to a hydrofinishingzone.
 32. The process of claim 28 wherein a stripping gas is used in thefirst intermediate separation zone to assist in recovering the mixturecontaining the light Fischer-Tropsch fraction and the hydrogen-rich C₄minus fraction.